Why do people think they need to protect children against s in particular?

You're right, it is not a gay/hetero issue. Unfortunately, there are more doented cases of Pedophile men going after little boys than girls. For some reason, most, or at least a large portion of, Pedophiles go after little boys. That's why more people worry about that. That's not to say others don't go after little girls, that is a concern as well.